(1) Field of the Invention
This invention relates to a resin composition capable of yielding a cured material excellent in heat resistance, and more particularly, it relates to a curable resin composition capable of providing varnish of improved workability and stability as well as formed products of excellent curability and physical and chemical properties.
(2) Description of the Prior Art
Maleimide-type resins are known as resins having excellent resistance to heat and are used, for example, in the form of polymaleimide resin which is a homopolymer of a maleimide compound or polymaleimide-polyamine type resin which is a copolymer with an amine. These maleimide-type resins can generally provide satisfactory properties with respect to resistance to heat. However, prior to polymerization, such maleimide compounds have a high melting point and, moreover, can be hardly dissolved in an organic solvent commonly used for the production of varnish and laminates. Due to the above-described nature of maleimide compounds, they are accompanied by a drawback that they must be dissolved in a polar solvent having a high boiling point and high hygroscopicity prior to their application. Thus, an impregnating varnish may be prepared from such a maleimide compound by using a polar solvent represented by N-methyl-2-pyrrolidone, N,N-dimethylacetamide, or N,N-dimethylformamide. Use of such a solvent however is not preferable as it is not only expensive but also susceptible of penetrating through the skin and absorbed in the human body, in other words, it is very toxic.
Furthermore, when a laminate is produced with varnish containing such a solvent, the solvent will become a main cause for deteriorating the properties of the laminate as it tends to remain in the final product to be obtained upon heating and curing same.
Where a usual maleimide-type resin is employed for the production of a copper-lined laminate, the adhesion between a copper foil and the substrate is not always good and its workability such as blanking work is not satisfactory.
On the other hand, epoxy resins are extensively used as various electric insulators, molded products, adhesive and paint since they can generally afford, upon curing same with a wide variety of curing agents, cured products excellent in electrical characteristics, mechanical characteristics, dimensional stability and chemical resistance.
However, despite having such excellent properties, conventional epoxy resins are considered to be unsatisfactory with respect to resistance to heat. Thus, varied methods have been taken into consideration with a view toward enhancing their resistance to heat.
To obtain a resin excellent in varied properties and assuring big commercial utility by maintaining the advantageous features of these two resins and simultaneously compensating for their drawbacks, it has been attempted to incorporate a maleimide compound in epoxy resin. However, it was very difficult to obtain a uniform and stable composition which is suitable for actual use, as conventional general maleimide compounds are considerably poor in their miscibility with epoxy resins and, for example, when formulated into varnish using a solvent, such varnishes are susceptible to separation and, when employed for solventless casting, the maleimide compounds tend to deposit as crystals.